The present invention relates to a method of manufacturing a semiconductor device and, more specifically, to a technique of forming an insulation film, such as a fluorine-added silicon oxide (SiOF) film and a fluorine-added nitrified silicon oxide (SiONF) film, on a semiconductor substrate by plasma CVD (chemical vapor deposition).
In conventional semiconductor devices, an SiO.sub.2 film is used as an insulation film for electrically isolating elements and wirings. The SiO.sub.2 film is formed chiefly by reduced-pressure CVD or atmospheric-pressure CVD using gas such as SiH.sub.4 and tetraethoxysilane (TEOS) as a raw stock. In particular, plasma CVD using crude gas such as TEOS and O.sub.2 is employed a lot to form an SiO.sub.2 film (TEOS film) since it can be formed at low temperature of about 400.degree. C. Since, in the CVD, high-purity crude gas is used as a reaction source more frequently than in another thin-film forming method, a high-quality film can be obtained.
Concerns about a delay in signal transmission have recently been rising in semiconductor devices. This is because a capacitance between wirings is increased by narrowing an interval between them due to miniaturization of elements. The delay in signal transmission is one factor in preventing an improvement in the performance (e.g., operation speed) of a semiconductor device. It is thus necessary to make the dielectric constant of an insulation film formed between the wirings as low as possible.
In semiconductor devices, it is required that a disconnection of wirings be prevented and a focus margin be improved. It is thus necessary to fill a space between high-density wirings without any void and to lessen a level difference between the surfaces thereof.
For the above reason, the development of a method of forming an insulation film capable of mitigating a level difference is demanded in manufacturing of semiconductor devices.
Recently, an SiOF film has been known as an insulation film capable of decreasing in dielectric constant; however, it has a problem of hygroscopicity. It has conventionally been reported that the SiOF film decreases in dielectric constant and increases in hygroscopicity as fluorine increases in concentration. The increase in hygroscopicity makes the dielectric constant high, causes corrosion of metal wiring due to liberation of hydrogen fluoride (HF), causes the film to come off, and decreases in reliability.
The relative dielectric constant .epsilon. of silicon oxide is 4.1 and decreases to 3.4 as the fluorine concentration (10 to 11 atom %) of the SiOF film increases. If the relative dielectric constant .epsilon. decreases further, the silicon oxide becomes useful in lessening a delay in signal transmission. The further decrease in relative dielectric constant .epsilon. heightens the fluorine concentration of the SiOF film, and the high fluorine concentration increases the hygroscopicity of the SiOF film, resulting in deterioration of characteristics of a semiconductor device. The lowest limit of the relative dielectric constant .epsilon. is therefore 3.4.
It is understood that the hygroscopicity of an SiOF film is increased by the remaining amount of impurities therein, especially hydrogen (H). To use the SiOF film as an insulation film, the hydrogen impurities remaining therein should be reduced as much as possible.
A technique of forming an SiOF film using HDP (high density plasma)-CVD whose plasma density is 10.sup.10 /cm.sup.3 or higher, is now under development. For example, a technique using an HDP source of ECR (electron cyclotron resonance) is reported and so is a technique using an HDP source such as an ICP (inductively coupled plasma) coil and helicon. Since, in the HDP-CVD, the resolution of crude gas is increased, the amount of hydrogen impurities contained in the SiOF film is smaller than that in an SiOF film formed by the conventional parallel-plate CVD; accordingly, the SiOF film is prevented from increasing in hygroscopicity.
FIG. 1 schematically shows the constitution of a prior art semiconductor device. In this device, an interlayer insulation film of low dielectric constant such as an SiOF film is buried into a space between two wirings.
As shown in FIG. 1, element isolation regions 102 each having an STI (shallow trench isolation) structure are selectively formed in a P-type silicon semiconductor substrate 100. An NMOS transistor is formed in each of element regions of the semiconductor substrate 100 which are delimited by the element isolation regions 102.
The NMOS transistor includes source and drain regions 104 and 104 formed in a surface area of the semiconductor substrate 100, a gate oxide film 106 formed on the surface of the substrate 100, and a gate electrode 108 formed on the gate oxide film 106 and between the source and drain regions 104 and 104.
An interlayer insulation film 110 of a BPSG (boron-doped phospho-silicate glass) film is formed on the semiconductor substrate 100 so as to coat the respective NMOS transistors. The film 110 includes a contact plug 112 which contacts one of the source and drain regions 104 and 104.
The surface of the interlayer insulation film 110 is flattened by CMP (chemical mechanical polishing), and a plurality of first metal wirings 114 each having a given pattern are formed thereon. Each of the first metal wirings 114 is constituted of an aluminum (Al) film 114a, and barrier metal layers 114b and 114c of Ti/TiN between which the film 114a is interposed. The surface of the interlayer insulation film 110, including the periphery of each metal wiring 114, is covered with a wiring protection film 116 having a uniform thickness and constituted of silicon oxide (SiO.sub.2).
A low-dielectric-constant interlayer insulation film 118 of an SiOF film is formed so as to coat the wiring protection film 116. The film 118 includes a contact plug 120 which passes through the wiring protection film 116 and contacts one of the first metal wirings 114.
The surface of the interlayer insulation film 118 is flattened by CMP and a plurality of second metal wirings 122 each having a predetermined pattern are formed thereon. Like the first metal wirings 114, the second metal wirings 122 are each constituted of an Al film 122a and barrier metal layers 122b and 122c of Ti/TiN between which the Al film 122a is interposed. The surface of the interlayer insulation film 118, including the periphery of each metal wiring 122, is covered with a wiring protection film 124 having a uniform thickness and constituted of silicon oxide (SiO.sub.2).
A low-dielectric-constant interlayer insulation film 126 of an SiOF film is formed so as to coat the wiring protection film 124.
In order to protect the semiconductor device, three or more metal wirings can be formed on the SiOF film 126, or a surface protecting insulation film (e.g., a passivation film) can be formed in place of the SiOF film 126. If the surface protecting insulation film is formed, the above wiring protection film 124 is not needed.
If an SiOF film is formed directly on a metal wiring by HDP-CVD, a corner portion of the metal wiring is damaged by sputtering damage to the metal wiring and reaction of fluorine (F) radical at the beginning of film formation. To resolve this problem, conventionally, all wirings are covered with a wiring protection film (SiO.sub.2 film) having a thickness of several tens of nanometers by the parallel-plate CVD.
If, however, the wirings are covered with the wiring protection film, the aspect ratio increases when the SiOF film is formed and thus the wiring-to-wiring burying characteristics deteriorate.
FIG. 2 is an enlarged view of the major part of the semiconductor device shown in FIG. 1. If a distance D between adjacent metal wirings 114 and 114 is decreased in accordance with higher-degree of integration of the semiconductor device, a wiring-to-wiring aspect ratio A (=H/D:H is a height of the metal wiring 114 or a distance from the bottom to the top of the metal wiring 114 or a thickness of the metal wiring 114). There is a problem in which a void 128 is easy to form when the low-dielectric-constant interlayer insulation film 118 is formed since the aspect ratio A is increased further by forming the wiring protection film 116.
If the wiring protection film 116 is formed, a distance between adjacent wirings, which is indicated by D', becomes shorter than the distance D by 2Y (Y=the thickness of film 116), and the height of the wiring, which is indicated by H', is almost equal to the height H if the thickness of film 116 is uniform (X=Y). The aspect ratio A' is therefore represented as H'/D' after the wiring protection film 116 (SiOF film) is formed.
In other words, the height H' of each metal wiring 114 after the wiring protection film 116 is formed on the surface of the metal wiring 114 is substantially equal to the height H thereof before the film 116 is formed. Therefore, the aspect ratio A' after the SiOF film is formed is greater than the aspect ratio A (A'&gt;A) by 2Y (Y=the thickness of the film 116 formed on the side of the metal wiring 114).
There is another problem in which a high-dielectric-constant SiO.sub.2 film (whose relative dielectric constant .epsilon. is 4.1) is formed as the wiring protection film 116 on the side walls of the metal wirings 114 and on the exposed surface of the interlayer insulation film 110 to thereby increase the capacitance between the wirings.
The above problems are not limited only to the first metal wirings 114. The same is true of the second metal wirings 122 and three or more metal wirings.